1. Field of the Invention
The present invention relates to an automatic sheet supplying apparatus which is incorporated into a recording system or an image reading system such as a facsimile, laser beam printer, image reader the like.
2. Related Background Art
As one of automatic sheet supplying apparatuses incorporated into an image reading system such as a facsimile, a sheet supplying apparatus comprising a sheet separating means including a separation roller and a friction member urged against the separation roller with a predetermined pressure, and a sheet supply means including a sheet supply roller and a driven roller urged against the sheet supply roller with a predetermined pressure has been known.
FIGS. 6 and 7 show an example of a conventional automatic sheet supplying apparatus of the above-mentioned type.
In FIGS. 6 and 7, the reference numeral 1 denotes an original support on which original sheets 2 are stacked; 3 denotes a sheet separating means disposed at a downstream side of the original support 1; 5 denotes a sheet supply means disposed at a downstream side of the sheet separating means 3; 6 denotes a sheet ejecting means disposed at a downstream side of the sheet supply means 5; and 7 denotes an image sensor of contact type disposed below an upper original guide plate 9 between the sheet supply means 5 and the sheet ejecting means 6.
An original presence/absence detection sensor 10 for detecting the presence or absence of the original sheet 2 stacked on the original support 1 is provided in connection with the original support 1. The sheet separating means 3 is constituted by a separation roller 11 and a friction plate (friction member) 12 urged against the separation roller 11 with a predetermined pressure. The friction plate 12 is biased by a separating and urging spring (coil spring) 13 toward the separation roller. The sheet supply means 5 is constituted by a sheet supply roller 15 and a driven roller 16 urged against the sheet supply roller 15 with a predetermined pressure. The driven roller 16 is biased by a plurality of leaf springs 17, 19 toward the sheet supply roller. The sheet ejecting means 6 is constituted by an ejector roller 20 and a driven roller 21 urged against the ejector roller 20 with a predetermined pressure. The driven roller 21 is biased by a plurality of leaf springs 17, 19 toward the ejector roller. The ejector roller 20 and the driven roller 21 of the sheet ejecting means 6 are made of material same as those of the sheet supply roller 15 and the driven roller 16 of the sheet supply means 5, respectively.
The leaf springs 17, 19 extend between and are rested on shafts 22, 23 of the driven rollers 16, 21 so that they apply the same pressure to the roller shafts 22, 23 by a weight 24 disposed at central portions of the leaf springs.
A rotational force from a motor (not shown) is transmitted to the separation roller 11 of the sheet separating means 3, the sheet supply roller 15 of the sheet supply means 5 and the ejector roller 20 of the sheet ejecting means 6. In this case, the rotational force of the motor is firstly transmitted to the sheet supply roller 15, and the rotational force transmitted to the sheet supply roller 15 is transmitted to the separation roller 11 and the ejector roller 20.
More particularly, as shown in FIG. 7, a gear 26 for transmitting the rotational force of the motor is attached to one end of a roller shaft 25 of the sheet supply roller 15. A gear 30 connected to the gear 26 through an intermediate gear 29 is attached to one end of a roller shaft 27 of the separation roller 11. Further, pulleys 32, 33 having the same diameters are attached to one end of the roller shaft 25 of the sheet supply roller 15 and one end of a roller shaft 31 of the ejector roller 20, these pulleys being interconnected by an endless belt 35. Thus, when the motor is rotated, the rotational force of the motor is transmitted to the sheet supply roller 15 via the gear 26. Further, the rotational force transmitted to the sheet supply roller 15 is transmitted to the separation roller 11 via the gear 26, intermediate gear 29 and gear 30, and, at the same time, is transmitted to the ejector roller 20 via the belt 35. Incidentally, the motor is drivingly rotated when the original presence/absence sensor 10 detects the original sheet 2 stacked on the original support 1.
With the arrangement as mentioned above, when the original sheets 2 are rested (stacked) on the original support 1 as shown in FIG. 6, by detecting the original sheet by means of the original presence/absence sensor 10, the motor (not shown) is rotated, thus rotating the separation roller 11, sheet supply roller 15 and ejector roller 20 in directions shown by the arrows simultaneously. In this way, as shown, the original sheets 2 rested on the original support 1 with being abutted against the separation roller 11 and the friction plate 12 are separated in such a manner that only a lowermost original sheet is separated by the separation roller 11 and is fed toward a downstream side of the sheet separating means 3.
In the sheet separating means 3, since the relation between a friction force .mu..sub.1 (between the separation roller 11 and the original sheet 2) and a friction force .mu..sub.2 (between the friction plate 12 and the original sheet 2) and a friction force .mu..sub.3 (between the original sheets 2) is so selected as to be .mu..sub.1 &gt;.mu..sub.2 &gt;.mu..sub.3, a plurality of originals 2 can be separated in a nip between the separation roller 11 and the friction plate 12 so that only the lowermost original sheet is separated by the separation roller 11 and is fed.
When a leading end of the original sheet 2 fed by the separation roller 11 is pinched by a nip 37 between the sheet supply roller 15 and the driven roller 16, the original sheet is conveyed by the sheet supply roller 15 toward a downstream side of the sheet supply means 5. Now, when the leading end of the original sheet 2 has passed through the nip 37, the leading end of the original sheet 2 is detected by an original end detection sensor 39, with the result that the original sheet 2 is fed by a predetermined amount. Thereafter, image information on the original sheet 2 is read by the image sensor 7 while the original sheet is being fed by the sheet supply roller 15. When the leading end of the original sheet is pinched by a nip 40 between the ejector roller 20 and the driven roller 21, the original sheet is conveyed without any slack by means of the ejector roller 20 and the sheet supply roller 15 which are rotated at the same speed. When a trailing end of the original sheet 2 has passed through the nip 37 between the sheet supply roller 15 and the driven roller 16 and is detected by the original end detection sensor 39, the original sheet is conveyed by the ejector roller 20 by a predetermined amount.
By the way, the above-mentioned conventional automatic sheet supplying apparatus is so designed that, immediately after the trailing end of the firstly fed original sheet 2 has passed through the nip 36 between the separation roller 11 and the friction plate 12, a next original sheet 2 rested on the original support 1 is supplied with following the firstly fed original sheet 2. In this case, if there is no gap (clearance) between the trailing end of the firstly fed original sheet 2 and the leading end of the next original sheet 2, the original end detection sensor 39 cannot correctly detect the trailing end of the firstly fed original sheet 2 and the leading end of the next original sheet 2, with the result that the image information cannot be correctly read by the image sensor 7 and/or the original sheet cannot be correctly ejected.
For these reasons, the conventional automatic sheet supplying apparatus is so designed that there is a difference in sheet feeding speed between the previous original sheet 2 and the next original sheet 2 by setting a peripheral speed of the sheet supply roller 15 to be faster than that of the separation roller 11. In this way, since the previous original sheet is conveyed by the faster sheet supply roller 15 and the next original sheet 2 is conveyed by the slower separation roller 11, a gap is created between the trailing end of the previous original sheet 2 and the leading end of the next original sheet 2.
However, in the conventional case where the peripheral speed of the sheet supply roller 15 is set to be faster than that of the separation roller 11 to create the gap between the trailing end of the previous original sheet 2 and the leading end of the next original sheet 2, the following problem arises.
That is to say, when the leading end of the supplied original sheet 2 is pinched by the nip 37 between the sheet supply roller 15 and the driven roller 16 and, thus, the sheet supply roller 15 starts to convey the original sheet 2, there arises a difference in speed between the separation roller 11 rotated at a speed slower than that of the sheet supply roller 15 and the original sheet 2 contacted by the separation roller 11 and conveyed by the sheet supply roller, with the result that a sheet portion contacted by the separation roller 11 is pulled by the faster sheet supply roller 15. Consequently, the separation roller 11 applies a load to the original sheet 2 being fed (i.e., there is a slip between the separation roller 11 and the original sheet 2). Accordingly, the image information recorded by a pencil and the like on a surface of the original sheet 2 which is contacted by the separation roller 11 will be deteriorated due to the relative slipping movement between the separation roller 11 and the original sheet 2.
In order to solve such a problem, a one-way clutch 41 of needle type is provided on the gear 30 attached to the roller shaft 27 of the separation roller 11 so that, when the original sheet 2 is fed at a speed greater than the peripheral speed of the separation roller 11, the peripheral speed of the separation roller 11 becomes the same as the speed of the original sheet 2. However, the one-way clutch 41 of needle type is very expensive, and does not permit to reduce a pitch circle of the gear 30 due to the outer configuration of the clutch, and, accordingly, the roller shaft 27 is lengthened, thus limiting the applications of the apparatus. Further, when the one-way clutch is used, if outer diameters of the separation roller 11 and the sheet supply roller 15 are preselected, the numbers of teeth of the gears associated with the separation roller 11 and the sheet supply roller 15 must be altered in order to control the gap between the previous original sheet 2 and the next original sheet 2, which is very difficult in design.
In place of the one-way clutch of needle type, a spring clutch can be used. However, since the spring clutch has a disadvantage that it generates the noise and/or slip if an amount of grease applied to the clutch is not proper, the control of the spring clutch is very troublesome.
As mentioned above, in the conventional method for creating the gap between the trailing end of the previous original sheet 2 and the leading end of the next original sheet 2 by setting the peripheral speed of the sheet supply roller 15 to be greater than that of the separation roller 11, there arose the problem that the image information on the original sheet 2 was deteriorated due to the relative slip between the original sheet and the separation roller 11. Further, the one-way clutch for solving the above problem also arose other various problems.